Integration of color and light intensity is what the eye needs to optimally entrain

Marijke Gordijn from Chrono@Work co-authors a paper, published in Scientific Reports, about how the eye uses natural light information to synchronize the biological clock to the outdoor light, even when clouds are covering the sky. The mammalian circadian system encodes both absolute levels of light intensity and color to phaselock (entrain) its rhythm to the 24-h solar cycle. The evolutionary benefits of circadian color-coding over intensity-coding per se are yet far from understood. A detailed characterization of sunlight is crucial in understanding how and why circadian photoreception integrates color and intensity information. To this end, we continuously measured 100 days of sunlight spectra over the course of a year. Our analyses suggest that circadian color-coding may have evolved to cope with cloud-induced variation in light intensity. We proceed to show how an integration of intensity and spectral composition reduces day-to-day variability in the synchronizing signal (Zeitgeber). As a consequence, entrained phase angle of the circadian clock will be more stable, which will be beneficial for the organism. The presented characterization of sunlight dynamics may become important in designing lighting solutions aimed at minimizing the detrimental effects of light at night in modern societies.